Certain radioactive materials, including plutonium, emit radiation and/or subatomic particles. Plutonium-239 and Plutonium 240, for example, emit gamma radiation and neutrons from spontaneous fission events. Detecting plutonium can be very challenging, and often requires specialized scientific instruments that are designed to detect neutrons. However, detecting free neutrons can be rather challenging since they do not carry an electrical charge.
Detectors have been made that can indirectly measure emitted free neutrons. For example, charged particles (tritium and protium) are produced when helium 3 absorbs a neutron, and the charged particles can be detected. Neutron detectors utilizing helium-3 have been proposed for detecting smuggled plutonium in shipping containers, but a worldwide shortage of helium-3 has prevented this to some extent. Alternate materials are now being explored to provide a substitute for helium 3.
Neutron detector tubes typically operate with a large voltage potential between a cathode and an anode, and therefore, a means of electrical isolation is usually required between the cathode and anode. The detector tubes are typically sealed to contain a fill gas. Conventional isolation and sealing methods have included ceramic to metal seals, which require tight tolerances and brazing operations that can add significant costs to each tube. Often, the neutron detector tubes are used in arrays consisting of multiple detector tubes, so any costly process or material used in the assembly of each tube can result in a significant expense in the final array.